Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as an energy source. Among other things, a great deal of research and study has been focused on lithium secondary batteries having high-energy density and high-discharge voltage. These lithium secondary batteries are also commercially available and widely used.
Generally, the lithium secondary battery is comprised of a structure having an electrode assembly composed of a cathode and an anode and a porous separator interposed between the cathode and anode, wherein the cathode and anode have an electrode material containing an active material is applied on a current collector, and impregnation of the electrode assembly with a non-aqueous electrolyte containing a lithium salt. As examples of the cathode, the use can be made mainly of lithium-cobalt oxides, lithium-manganese oxides, lithium-nickel oxides, lithium composite oxides, or the like, whereas for the anode, the use can be made mainly of carbonaceous materials.
Such a lithium secondary battery, for example, when the water content increases inside the battery, the degradation of an electrolyte is induced thereby producing acids. Thus produced acids facilitate a side reaction such as degradation of solid electrolyte interface (SEI) at the anode or dissolution of a cathode active material. Ultimately, problems such as reduction of a battery capacity and increase in the internal resistance are induced. That is, since the performance of a lithium secondary battery is influenced greatly by the water content inside the battery, it is most important to prevent the water from penetrating into the battery during the battery production.
Therefore, in the present invention, hydrophobic particles are contained in the electrode material so as to exert hydrophobicity throughout the constituent components of the battery. Thereby, the present invention provides a technique to minimize absorption and inflow of water during the battery production.
In this connection, Japanese Patent Laid-Open Publication No. 2001-093498 discloses a technique for producing a separator by mixing a polyolefin resin and an inorganic powder whose surface is hydrophobized with chlorosilane or silazane. In this technique, the inorganic powder whose surface is hydrophobized as in the above is added to the separator so as to complement hydrophilicity of the inorganic substance included for preventing the separator from excessively melting upon exerting the shut-down function. However, the separator material itself is usually hydrophobic. Thus, unless a hydrophilic inorganic substance is further added to the separator, the technique for giving hydrophobicity to the separator is not required.
Additionally, Japanese Patent Laid-Open Publication No. 2002-015728 discloses a technique for applying an electrode material on an electrode current collector, and then forming a hydrophobic substance layer on the surface of the electrode material in order to inhibit water from penetrating into the electrode material. However, there is a limit to this technique in that it is applied to the battery production only when the process is carried out in a very low humidity condition or completely no humidity condition. That is, in this technique, penetration of water is inhibited by the hydrophobic substance layer applied on the surface of the electrode material during the battery production after applying the electrode material on the electrode current collector. Therefore, there is a problem in that water absorbed or flowed into the electrode material during the production or applying process of the electrode material is rather unable to evaporate due to the hydrophobic substance layer.
As a technique for removing water contained in an electrode material, Korean Patent Laid-Open Publication No. 1995-0002099 suggests a method for sintering an electrode material after applying the electrode material on an electrode current collector. However, in this technique, various substances such as a binder constituting the electrode material may undergo degradation reaction during the sintering. Thus, it is not preferable to apply this technique to the battery production.